Super Metroid had two points in the game where you needed to roll into a ball and rest in the hands of a bird statue. Nothing like this existed in Metroid before, and there was no hint that you had to do it. Also, the first statue was extraordinarily out of the way and hard to find.
TRY TO GET THAT GRAVITY SUIT WITHOUT A WALKTHROUGH!
The game as a whole wasn't too cryptic, but tossing in something brand-new like that was just rough.